Damping for dynamic loudspeakers



Nov. 29, 1949 k 2,489,862

DAMPING FOR DYNAMIC LOUDSPEAKERS Filed Jan. '7, 1943 Inventor- El lsworth D. Cook,

torney.

Patented Nov. 29, 1949 DAMPING FOR DYNAIWIC LOUDSPEAKERS Ellsworth D. Cook, Scotia, N. Y., assignor to General Electric Company, a. corporation of New York Application January '7, 1943, Serial No. 471,575

Claims. 1

My invention relates to acoustic devices comprising vibratory elements of the cone diaphragm type, particularly to methods of, and means for, preventing undesired resonance effects in operation of such devices, and its object is to provide improved damping means which operate effectively to better the response characteristic of the devices at and near the point at which resonance occurs, without impairing the overall response.

It is known that acoustic devices of the dynamic loudspeaker type are in general not adequately damped. The output of the speaker at a given short time period often represents a mixture of the signals corresponding to frequencies desired at this period with a signal corresponding to a frequency which should have been completed at a previous period. Such undesirable effects might occur, for example, from a transient response caused by some form of excitation, permitting the diaphragm to continue to vibrate at its resonant frequency for an appreciable period after the excitation ceased.

It will be recognized that the cone structure of the dynamic speaker at its natural period of vibration, of the order of 100 cycles per second, is essentially a simple tuned mechanical circuit. To remove the undesirable resonance effects above noted, this tuned mechanicalcircuit requires a definite amount of damping, but loading of the circuit thus constituted by the cone structure by a damping resistance is undesirable unless the damping resistance may be removed from the circuit at all frequencies except those at which resonance may occur. Such 'a requirement is difficult, if not impossible, to meet with any simple form of mechanical resistance merely added to the existing mechanical circuit.

In accordance with my present invention. the above described undesirable resonance effects are overcome by providing an additional or auxiliary circuit which is elastically coupled to and decoupled from the cone structure in accordance with the frequency. For this purpose, the aux liary circuit is so arranged as to mass and stiffness that it is tuned to resonance at the natural period of the mechanical circuit represented by the cone structure, and damping is provided in this auxiliary tuned circuit. The coupling of the auxiliary tuned and damped mechanical circuit to the mechanical circuit represented by the cone structure, comprising the cone and its operating winding and suspension means, is so arranged that the auxiliary circuit is effectively decoupled from the cone structure at all frequencies above the resonant period of the mechanical circuit 2 represented by the latter structure. In practice, it is sufiicient that the decoupling take place at the resonant frequency since the natural period of the cone structure ordinarily occurs approximately at the lower limit of the frequency range over which the loud-speaker system operates.

In one form of my invention the auxiliary damped tuned vibratory system may include a mass together with resilient means connecting the mass to the vibratory system constituted by the cone structure, damping means being associated with the mass. The motion of the mass itself at the resonant frequency of the cone structure then occasions a power loss in some form of mechanical resistance which is provided, associated with the mass. This mechanical resistance may be a dash-pot, or some form of viscous damping means such as oiled felt, loaded rubber, also a material known under the trade name of Viscoloid, or in certain designs of vanes which might be used to employ air friction. The above mentioned mass itself may be constituted by a conductive element, for example a metal member arranged for vibratory motion in a magnetic field at the resonant frequency of the cone structure, the separate mechanical resistance such as a dash pot or a viscous damping means being then dispensed with.

In a modification of my invention, the damping system may be simplified by connecting an annular conductive member to the rear of the operating winding of the diaphragm through an elastance such as a fold of a flexible Sylphonlike construction, and arranging the loudspeaker field structure in such manner that the annular member vibrates in a magnetic field.

The novel features which are considered to be characteristic of my invention are set fo th with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanyi g drawing wherein Fig. 1 is a cross-sectional fragmentary view of an acoustic device in which my invention has been embodied, and Fig. 2 is a similar view of a modification of my invention.

In Fig. 1 showing an acoustic device which for illustrative purposes is shown as a usual dynamc loudspeaker, the numeral l0 designates a vibratory system con tituted by a cone structure comprising a cone diaphragm l l, suspension means therefor in the form of a usual flexible ring l2 which may be secured to a fixed frame element (not shown), and an operating winding I3, which, if the acoustic device is employed for converting electrical oscillations into sound, is a usual voice coil which may be connected to a suitable oscillation source (not shown). The voice coil is mounted in a usual manner on a cylindrical support l4 connected to the diaphragm II to vibrate in an air gap l5 between the poles l6 and l! of a magnet, only the poles l6 and ll of which are shown in the drawing.

In the embodiment illustrated in Fig. 1, a damping system designated generally by the numeral I3 is provided for preventing undesired sound effects otherwise tending to be produced because of continued vibration of the cone structure Ill at its resonant frequency after the eX- citation impressed on the voice coil l3 at this resonant frequency has ceased, thereby causing a mixing of this unduly prolonged resonant frequency with the desired signal frequencies which are impressed on the voice coil IS in the immediately following period. The damping system l8 in Fig. 1 in accordance with my invention is constituted by an auxiliary vibratory system comprising a mass element, or preferably of a plurality of masses I9, operatively associated with elastic elements 20 which may be springs of suitable form. The masses l9 are connected elastically as by the springs 26 to the cone structure ID, for example to the cylindrical support 14 for the voice coil l3.

Mechanical resistance means is provided operatively associated with the auxiliary vibratory system l8. In the embodiment illustrated in Fig. 1 the mechanical resistance is shown as comprising a form of viscous damping material such as members 2! of Viscoloid interposed between the masses l9 and fixed supporting members 22.

In operation of the acoustic device illustrated in'Fig. 1, masses I9 and elastances 2B of the auxiliary mechanical vibratory circuit I8 are so proportioned and arranged that the auxiliary circuit is tuned to the natural period, for example 100 cycles per second, of the mechanical vibratory circuit constituted by the cone structure l0. Therefore when the cone structure vibrates at its natural period thus tending to produce the unwanted resonance effects above mentioned, energy is absorbed from the cone structure by the auxiliary circuit l8 which is tuned not consume energy, at these latter frequencies,

absorbed from the main mechanical vibratory circuit constituted by the cone structure.

In case the coefiicient of coupling between the two mechanical vibratory circuits Ii: and I 8 is not of suitable value, the coupling may be controlled by connecting the circuit l 8 to some other region of the circuit In than to a point adjacent the winding l3, as to a suitable region of the diaphragm II for example.

In Fig. 2 the modification illustrated therein 1 comprises, as in Fig. 1, a diaphragm II and an operating coil I3 mounted upon a cylindrical 19- port I4 and arranged to vibrate in an air gap [5 between poles l6 and ll of a magnet. In the device of Fig. 2, however, the auxiliary vibratory circuit corresponding to auxiliary circuit I8 of Fig. 1 comprises a conductive mass in the form of an annular member 23 of copper, for example, arranged to vibrate in a magnetic field, as by mounting the copper ring in an air gap 24 between central pole l1 and an auxiliary pole 25 of a magnet. The mass, copper ring 23, is connected elastically to the cone structure preferably by an elastance element in the form of a cylindrical support 26 having a single flexible fold 21 similar to a fold of a Sylphon bellows construction and connected to the support I4 of voice coil I3.

In operation of the modification illustrated in Fig. 2, the auxiliary mechanical vibratory circuit comprising the mass constituted by the conductive annular member 23 is so arranged that this auxiliary circuit is tuned to the natural period of the main mechanical vibratory circuit constituted by the cone structure. The action of the auxiliary vibratory system in preventing the above-mentioned undesirable resonance effects is the same as described in connection with the similar system of Fig. 1 except that in the system of Fig. 2 the vibratory motion of the mass, constituted by the conductive ring 23, is damped by the eddy current losses occasioned by the mounting of the ring in a magnetic field.

It will be seen that I have provided, in an acoustic or like device having a main mechanical vibratory circuit and characterized in its operation by disturbing resonance effects at and near its natural period, an auxiliary damping circuit tuned to the resonance frequency of the main circuit and coupled tightly enough thereto to absorb therefrom a desired amount of vibratory energy at the resonance frequency, sufiicient, for example, to suppress the disturbing resonance effects.

My invention has been described herein in different embodiments for purposes of illustration.

. It will be understood, however, that the invention is susceptible of various changes and modifications and that by the appended claims I intend to cover any such modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an acoustic device, a first moving system comprising a cone diaphragm having a central aperture, a cylindrical sleeve connected to said diaphragm at the inner periphery of said aperture, a substantially cylindrical voice coil sup ported on said sleeve, an auxiliary moving system tuned to the resonant frequency of said first named moving system, means for damping said J auxiliary system, and means for coupling elastically said auxiliary system to said first system, said auxiliary system including an electrically insulated concentrated conductive mass mechanically'connected to said sleeve and located at a point displaced radially from the axis of said sleeve by a distance at least equal to the radius of said sleeve, movement being imparted to said auxiliary system substantially only during vibration of said first system near said resonant frequency.

2. In an acoustic device, a first moving system comprising a cone diaphragm having a central aperture, a cylindrical magnet extending into said aperture, a cylindrical sleeve surrounding .3m m gnet and c nnected to said diaphragm at the inner periphery of said aperture, a substantlally cylindrical voice coil supported On said sleeve, an auxiliary moving system tuned to the resonant frequency of said first named moving system, means for damping said auxiliary system, and means for coupling elastically said auxiliary system to said first system, said auxiliary system including an electrically insulated concentrated conductive mass mechanically connected to said sleeve and located at a point displaced radially from the axis of said sleeve by a distance at least equal to the radius of said sleeve, movement being imparted to said auxiliary system substantially only during vibration of said first system near said resonant frequency.

3. In an acoustic device, in combination, means defining a magnetic circuit, a vibratory system comprising a diaphragm and a winding adapted to vibrate in a portion of the magnetic field of said magnetic circuit, a damped auxiliary vibratory system tuned to the resonant frequency of said first-named system and comprising a conductive mass adapted to vibrate in a portion of the magnetic field of said magnetic circuit, and means to connect elastically said mass to said first-named vibratory system.

4. In an acoustic device, in combination, means establishing a magnetic field, a vibratory system constituted by a cone structure comprising a diaphragm and an operating winding, a damped auxiliary vibratory system tuned to the resonant frequency of said first-named system and comprising an annular conductive element adapted to vibrate in said magnetic field, and

means for coupling elastically said conductive element to said cone structure at a point thereof adjacent to said winding.

5. In an acoustic device, in combination, means establishing a magnetic field, a vibratory system constituted by a cone structure comprising a diaphragm and an operating winding, a damped auxiliary vibratory system tuned to the resonant frequency of said first-named system comprising a metal ring adjacent to said winding and arranged to vibrate in said magnetic field, and an elastance element connecting said ring to said winding.

ELLSWORTH D. COOK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 989,958 Frahm Apr. 18, 1911 1,648,120 Harrison Nov. 8, 1927 1,658,349 Moore Feb. 8, 1928 1,734,944 Green Nov. 5, 1929 1,904,537 Round et al Apr. 18, 1933 1,952,167 Hosbrouck et al. Mar. 27, 1934 2,007,747 Ringel July 9, 1935 2,318,517 Olson May 4, 1943 FOREIGN PATENTS Number Country Date 548,278 Great Britain Oct. 5, 1942 709,205 France May 11, 1931 

